[C++20] [Modules] Only diagnose the non-inline external variable

definitions in header units

Address part of #60079.

Since the the declaration of a non-inline static data member in its
class definition is not a definition. The following form:

```
class A {
public:
    static const int value = 43;
};
```

should be fine to appear in a header unit. From the perspective of
implementation, it looks like we simply forgot to check if the variable
is a definition...

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D141905

clang/lib/Sema/SemaDecl.cpp

@@ -13088,6 +13088,7 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
   // C++ [module.import/6] external definitions are not permitted in header
   // units.
   if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
+      VDecl->isThisDeclarationADefinition() &&
       VDecl->getFormalLinkage() == Linkage::ExternalLinkage &&
       !VDecl->isInline()) {
     Diag(VDecl->getLocation(), diag::err_extern_def_in_header_unit);

clang/test/CXX/module/module.import/p6.cpp

@@ -22,3 +22,9 @@ int ok_var_decl;
 
 int bad_var_definition = 3;  // expected-error {{non-inline external definitions are not permitted in C++ header units}}
 
+class A {
+public:
+    // This is a declaration instead of definition.
+    static const int value = 43; 
+};
+